There is a system wherein a programmable controller performs the sequence control like that aforementioned above. In recent years, there is becoming widespread a PLC (so-called safety PLC) of an independent construction which is specialized for the safety management of the system in detecting the abnormality and the emergency state of the system as well as in automatically performing an emergency safety stop of the system. Patent Documents 1 and 2 noted below exemplify technologies relating to a so-called safety PLC like this.
As one of other technologies, FIG. 14 exemplifies a circuit diagram (in normal state) of a known input circuit 201 which is utilized in a programmable controller in the prior art. This input circuit 201 partly and briefly shows an input circuit in actual use. In the present circuit, switches SW1 and SW0 constituting an emergency stop switch are duplexed and are normally closed contacts. Photo couplers 10, 20, 30 and 40 are used in the operation state that electric current always flows therethrough. When the switches SW1, SW0 are brought into open states, the electric current flowing through the photo couplers 10, 20 and the electric current flowing through the photo couplers 30, 40 are cut off either. In this way, duplex constructions are provided by a circuit system for inputting and processing an input signal from the switch SW1 (the upper-half system of the illustrated input circuit 201 will be referred to occasionally as A-system) and by another circuit system for inputting and processing an input signal from the switch SW0 (the lower-half system of the illustrated input circuit 201 will be referred to occasionally as B-system).
Then, with the duplexed constructions, when the input signal detected in the A-system to indicate the open/close state of the switch SW1 coincides with the input signal detected in the B-system to indicate the open/close state of the switch SW0, the signals are treated as a normal input signal, and the subsequent logic processing is executed to control an output device. For example, when the depression of the emergency stop switch brings the contacts into open states, the two input signals outputted from the switches become signals both indicating the open states and come to coincidence, so that they are treated as a true input signal to execute an emergency stop processing for stopping the operations of all the output devices.
On the contrary, when the input signals from the switch SW1 and the switch SW0 of the two systems do not coincide, such is judged to be the occurrence of an abnormality, in which event an abnormality stop, that is, an emergency stop is performed to emergently stop all the output devices in a moment.
On one hand, the input circuits and the control devices therefor are always self-diagnosed. The method therefor will be described hereunder.
In FIG. 14, symbols (a), (b), (c) and (d) respectively denote inverters (logic reversers). When a diagnosis pulse OA (signal L in the figure) is outputted to the inverter (b) from an A-system control microcomputer (not shown) which controls the illustrated safety management, the electric potential at a diagnosis pulse input terminal 40a of the photo coupler 40 rises due to the reverse action of the inverter (b). Thus, since the emission of an LED built in the photo coupler 40 is discontinued temporarily for the period of the pulse (signal L) only, the normal current (in) flowing through the photo coupler 30 is once interrupted. As a consequence, a diagnosis result (response signal IB to the diagnosis pulse OA) which is inputted to a B-system control microcomputer (not shown) on the other side through the inverter (c) is temporally displaced from “1” to “0”.
FIG. 15-A shows this relation. That is, FIG. 15-A shows the relation between the diagnosis pulse OA and the response signal IB at the time when the aforementioned diagnosis pulse OA (signal L in the figure) is outputted with the terminals P and Q being not short-circuited therebetween and with both of the switches Sw1 and SwO being closed. The same symmetrical relation holds at the time when a diagnosis pulse OB is sent from the B-system control microcomputer conversely. FIG. 15-B shows the relation between the diagnosis pulse OB and the aforementioned response signal IA in FIG. 14. These relations can be understood easily in terms of the symmetry of the circuit.
For example, the aforementioned diagnosis wherein a sender side of the diagnosis pulse and a receiver side of the response signal to the diagnosis pulse are constructed by the different computers in this way will be referred to as cross-diagnoses hereafter. In the cross-diagnoses like this, however, between the microcomputers, there may be provided communication means or diagnosis result sharing means of the configuration that enables the diagnosis pulse sender side to make reference immediately to the aforementioned response signal received on the receiver side.
By conducting the cross-diagnoses like this, it can be realized to always monitor the conductive states between respective terminals (O, P, Q, R) and the operational state of the circuit through the complimentary cooperation, so that the duplexing and steady monitoring of the input circuit can be attained. Further, by making the diagnosis pulses cross mutually between the different systems, there can also be obtained an advantage that the control microcomputer of the opposing system is always monitored for the normal operation.
In the prior art system, in the aforementioned manner, the input circuit 201 is duplexed by duplexing the transmission line for switch open/close signals, switches themselves and so on as shown in FIG. 14, whereby the safeness of the system can be ensured.
Patent Document 1 is Japanese unexamined, published patent application No. 2004-46348, and Patent Document 2 is Japanese unexamined, published patent application No. 2002-358106.
However, when a short-circuit occurs between the input terminals P and Q to the same input device whose input circuit 201 is duplexed as shown in FIG. 16, it results that the photo couplers for the A and B-systems are connected directly, whereby there is generated a short-circuit current (is) shown in the figure. This causes electric current to continuingly flow through the respective photo couplers 10, 20, 30, 40 regardless of the open/close states of the switches SW1, SW0. As a result, even when the emergency stop switches SW1, SW0 are brought into open states, each of the aforementioned switch open/close signals (IA, IB) remains to indicate the closed states without changing. That is, both of them are always held to remain at “1”. As a consequence, the abnormality caused by the short-circuit like this cannot be detected even by the use of non-coincidence detection means which has been known for monitoring the non-coincidence between the response signals IA, IB duplexed as mentioned above.
When the duplexed switches are turned into the open states in the state that a short-circuit is taken place between input terminals for different input devices, the input signal to the system whose terminal has not been short-circuited is brought into the open state, while the input signal to the other system remains in the closed state. Thus, the duplexed signals are judged to be in non-coincidence, and an emergency stop processing is brought into operation immediately, so that no problem arises.
On the other hand, description will be described regarding the case that after the occurrence of the aforementioned short-circuit, the emergency stop switches SW1, SW0 are brought into the open states and then, the aforementioned cross-diagnoses are carried out. Actually, since the input circuit 201 as shown in FIG. 14 become needed by the number corresponding to the number of the devices to be controlled and peripheral devices therefor, the plurality of the input circuits 201 shown in FIG. 14 are arranged in parallel. In this case, the prior art safety PLC which is composed mainly of the aforementioned A-system control microcomputer and the B-system control microcomputer always monitors the safety of the system by repetitively executing the processing that the microcomputers read the states of respective input terminals at respective parts of plurality of the input circuits arranged in parallel in this way by cyclically outputting diagnosis pulses to each terminal.
In this case, since the circuit has been short-circuited, it results that electric current flowing through the short-circuited circuit in the other system is also interrupted by the diagnosis pulse, and this phenomenon can be detected by the change in level of the input signal. Accordingly, even when the non-coincidence detection circuit does not operate effectively, it becomes possible to detect the aforementioned short-circuit fault based on the cyclic diagnosis insofar as the cyclic diagnosis like this is always carried out.
However, since the cyclic diagnosis is carried out sequentially for the respective terminals, the diagnosis cycle at which the cyclic diagnosis is carried out is proportional to the number of objects to be diagnosed. Therefore, the cycle tends to become very long, as exemplificatively described later with reference to FIG. 6 and the like. Therefore, the present situation is that it is hard to say that a required time taken to detect the abnormality such as the aforementioned short-circuit can be shortened sufficiently.
Further, since the diagnosis cycle in the cyclic diagnosis is proportional to the number of the input terminals of the input circuits, it is likely that the problem like this will increasing become prominent and actual as the system becomes large in scale.
The present invention is made to solve the aforementioned problems, and the object thereof is to make it possible to perform a fault detection reliably and to perform an emergency stop more reliably and quickly.